Harley-Davidson is paying US authorities $12m (£9m) to settle allegations its motorbikes polluted the air at higher levels than allowed.

It has also been ordered to stop selling "super tuners", devices that enable motorbikes to generate power but also increases gas emissions.

US authorities said Harley had sold around 340,000 super tuners since 2008.

The company denied the claims, saying the devices were only sold for use in off-road or closed-course competitions.

The motorcycle-maker said the settlement was "a good faith compromise".

Harley will have to stop selling super tuners and destroy any remaining stock at its dealerships this month.

The US Justice Department has also ordered Harley to spend $3m on a local environmental project aimed at reducing air pollution.

"Given Harley-Davidson's prominence in the industry, this is a very significant step toward our goal of stopping the sale of illegal after-market defeat devices that cause harmful pollution on our roads and in our communities," the Justice Department's assistant attorney general John Cruden said in a statement.

The US Environmental Protection Agency has been cracking down on companies believed to be violating the federal Clean Air Act.

There has been increased scrutiny on emissions and the use of "defeat devices" after carmaker Volkswagen admitted to using illegal software to bypass US regulations.